Low friction hydrogel having straight chain polymers and method for preparation thereof

ABSTRACT

A low friction hydrogel, wherein a linear chain polymer is admixed with or graft-polymerized to a polymer gel; and a method for preparing the hydrogel. The hydrogel exhibits improved low friction property over a conventional material.

TECHNICAL FIELD

[0001] The invention relates to a low friction hydrogel having linear chain polymer and a method for preparation thereof.

BACKGROUND ART

[0002] In order to realize a mechanical motion under a low friction a ball bearing is used or a method to realize a sliding friction under the presence of lubricants such as silicon oil or glycerin is conventionally used. In the former case, the apparatus not only becomes complex, but there is a defect that the friction increases under low speed. In the latter case, due to detachment or elution of the lubricant a successive supply must be made, and therefore there is a problem in durability of effect. There has been almost no example of a low friction motion without a lubricant except the only one example using a ultra-high polymer polyethylene or Teflon, and in these cases it is difficult to make the frictional coefficient not more than 0.01.

[0003] In the meantime the inventors are the first in the world to report that polymer hydrogels are very low as 10⁻²-10⁻³ in the surface frictional coefficient compared with other solid materials [J. Phys. Chem. B, 101,5487-5489(1997), J. Cmem. Phys., 109, 8062-8068(1998), J. Phys. Chem. B, 103,6001-6006(1999), J. Phys. Chem. B, 103,6007-6014(1999), The Japan Academy, 75,122-126(1999), J. Phys. Chem. B, 104, 3423-3428 (2000)], and the elucidation of a friction mechanism in a biological joint and further the application to an artificial joint and the like are expected.

[0004] In JP, A, 10-500038 is described a hardening material containing a polymer matrix such as silicon polymer and a hydrogel, which is used for healing on an injury site such as the joint and for a surface finish. The hydrogel consists of a hydrophilic and water-insoluble polymer and reduces a surface friction force.

[0005] In JP, A, 8-19599 is described a medical device having in the surface a layer to form a hydrogel when swollen, which consists of a water-soluble and water-swollen polymer having a reactive functional group and of an antithrombotic drug. The hydrogel layer, which is fixed on the surface of a medical device such as catheter, becomes a lubrication layer and reduces the friction.

[0006] In JP, A, 6-71818 is described a underwater clothing small in a frictional resistance toward water, which uses a composite sheet consisting of a fiber base and a resin membrane containing a water-soluble alginate.

[0007] Thus, in the development of a medical device or the like materials in which the frictional resistance of a material surface is small have been developed, though in case of considering the application to an artificial joint and the like, the frictional coefficient of the joint is 0.001-0.03, and in a view point of a low friction material similar to a biological body or the realization of a low friction under a low velocity, a satisfactory material has not been obtained yet.

DISCLOSURE OF THE INVENTION

[0008] Consequently, the problem of the invention is to provide a further low friction material to satisfy the above requirements.

[0009] During extensive researches to solve the above problem the inventors noticed that on the surface of fishes or marine algae or in an internal organ a certain kind of polymer is excreted and this has a big role for the reduction of the resistance from water or the friction in case of swallowing foods, and found that a further low friction hydrogel can be obtained by involving a linear chain polymer into a polymer gel. As a result of further studies the invention has been accomplished.

[0010] Namely, the invention relates to a low friction hydrogel, wherein a linear chain polymer is admixed with or graft-polymerized to a polymer gel.

[0011] The invention also relates to the above low friction hydrogel wherein the linear chain polymer is graft-polymerized on a surface of the polymer gel.

[0012] Further, the invention relates to the above low friction hydrogel, wherein monomers constituting the polymer gel and monomers constituting the linear chain polymer are the same kind of monomers.

[0013] The invention also relates to the above low friction hydrogel, wherein the frictional coefficient is not more than 0.01.

[0014] Further, the invention relates to the above low friction hydrogel, wherein the content ratio of the linear chain polymer relative to the total weight of the low friction hydrogel is 2-300 wt. %.

[0015] The invention also relates to the above low friction hydrogel, wherein the polymer gel is an ionic gel.

[0016] Further, the invention relates to use of the above low friction hydrogel on surfaces of a solid and a biological tissue.

[0017] The invention also relates to a method for preparing the low friction hydrogel, wherein the linear chain polymer or monomers forming the linear chain polymer is admixed with and/or graft-polymerized to a polymer gel or monomers forming the polymer gel.

[0018] Further, the invention relates to the above method, wherein the linear chain polymer is admixed with and then graft-polymerized to the polymer gel.

[0019] The invention also relates to the above method wherein one or more species of monomers to form the linear chain polymer are admixed with and polymerized to the polymer gel, so as to involve the linear chain polymer into the polymer gel.

[0020] Further, the invention relates to the above method, wherein the linear chain polymer are admixed with and polymerized with one or more species of monomers forming the polymer gel, so as to involve the linear chain polymer into the polymer gel.

[0021] The invention also relates to the above method wherein one or more species of monomers forming the polymer gel and one or more species of monomers forming the linear chain polymer are admixed and polymerized, so as to graft the linear chain polymer to the polymer gel.

[0022] Further, the invention relates to the above low friction hydrogel, wherein monomers forming the polymer gel and monomers forming the linear chain polymer are the same type of monomers. The invention also relates to the method for preparing the low friction hydrogel, wherein monomers forming the polymer gel are polymerized on a hydrophobic substrate.

BRIEF DESCRIPTION OF DRAWING

[0023]FIG. 1 shows the results on the rate of rotation dependency of the friction force of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) gel against a glass plate.

[0024]FIG. 2 shows the results on the load dependency of the friction force of AMPS gel when the velocity is 0.01 rad/s.

[0025]FIG. 3 shows the results on the frictional coefficient of dimethyl acrylamide (DAMM) gel, AMPS gel and poly(vinyl alcohol) (PVA) gel against a glass plate.

[0026]FIG. 4 shows the results on the frictional coefficient of DAMM gel and AMPS gel against a glass plate

MODE FOR CARRYING OUT THE INVENTION

[0027] Although the monomers constituting the polymer gels used in the invention are not limited if they are monomers forming hydrogels with three-dimensional network atructure, typically illustrative are acrylic acid or methacrylic acid and a derivative thereof, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, acrylamide or methacrylamide and a derivative thereof, stylene sulfonic acid, vinyl sulfonic acid, vinyl phosphoric acid, vinylpyridine, trimethylvinylpyridinium chloride, 3-acryloylaminopropyltrimethylammonium chloride, 3-dimethylmethacryloyloxyethylammonium propanesulfonic acid and the like. Preferable are acrylamide, 2-acrylamide-2-methylpropanesulfonic acid, acrylic acid, stylene sulfonic acid, etc.

[0028] A crosslinking agent to crosslink these monomers is N,N′-methylenebisacrylamide, ethyleneglycol dimethacrylate, divinylbenzene or the like.

[0029] As the monomers forming the linear chain polymer used in the invention, any two or three components of the monomers described above can be used, though the same species of monomers as the monomers constituting the above polymer gels can also be used.

[0030] Further, except the above polymer gels can also be used polysaccharide gels such as gellan gel, kappa-carrageenan gel, agarose gel, carboxymethyl cellulose gel or the like, protein gels such as gelatin, collagen or the like, nucleic acid gels such as DNA, RNA or the like, and polymer gels such as polyvinyl alcohol, polyglutamic acid, polyethylene imine, frozen-thawed gel or the like.

[0031] The content ratio of the linear chain polymer relative to the total weight of the low friction hydrogel is preferably 2-300 wt. %., especially 5-100 wt. % from a viewpoint of the reduction effect of the friction force.

[0032] The low friction hydrogels of the invention include (1) the low friction hydrogels in which the linear chain polymer is admixed with the polymer gel, or (2) the low friction hydrogels in which the linear chain polymer is graft-polymerized to the polymer gel.

[0033] As the method for preparing the above (1) hydrogels, that is, the method for preparing the hydrogels in which the linear chain polymer is admixed with the polymer gel, any method may be used if it is a method in which the linear chain polymer can be admixed with the polymer gel, though typically, the following methods are illustrated.

[0034] a. The linear chain polymer is physically admixed with the polymer gel.

[0035] b. After forming the polymer gel, the polymer gel is fully immersed in a monomer solution forming the linear chain polymer, and the monomers are dispersed in the gel, followed by polymerization of the monomers.

[0036] c. In case of forming the polymer gel, the linear chain polymer is admixed with a material such as monomers and the gel is formed by pollymerization.

[0037] In the meantime, as the method for preparing the above (2) hydrogels, that is, the method for preparing the hydrogels in which the linear chain polymer is graft-polymerized to the polymer gel, any method may be used if it is a method in which the linear chain polymer can be graft-polymerized to the polymer gel, though typically, the following methods are illustrated.

[0038] d. The linear chain polymer is admixed with a formed polymer gel, followed by graft-polymerization.

[0039] e. One or more species of monomers forming the polymer gel and one or more species of monomers forming the linear chain polymer are admixed and polymerized, so as to graft the linear chain polymer to the polymer gel.

[0040] f. Polymerization of the polymer gel is made between hydrophobic substrates.

[0041] In particular, the above f method is a novel gel synthetic method which the inventors have developed originally (refer to J. Phys. Chem. B, 103,6069-6074(1999), Biomacromolecules, 1, 162-167(2000), Proceeding of The Society of Polymer Science, Vol.48, No. 10, 2603-2604(1999), Proceeding of The Society of Polymer Science, Vol.49, No. 12, 3689-3692(2000). In place of a gel polymerization conventionally carried out on a hydrophlic substrate such as a glass plate or the like, use of a hydrophobic substrate such as Teflon plate, polypropylene, polyethylene, polystylene, or the like produces a concentration gradient of a hydrophilic monomer solution near the hydrophobic substrate, and a crosslinking density becomes low, resulting to form gels having graft chains on the surface. This method is one step, does not need to use other reagents or the like, and is particularly preferable since a desired gel may simply be formed by the hydrophobic substrate only

[0042] Since the polymer gel having the linear chain polymer, which is obtained by the above each method, contains the hydrophilic polymer chains in the gel or on the surface, a water content of the gel is further increased, and the hydrated gel or the polymer chains hydrated on the surface work as a lubricious layer at the interface of solids, enabling to obtain a low friction polymer hydrogel. Further when a low friction hydrogel, which is an ionic gel, is applied to a solid surface in case of using an ionic straight polymer chain, an electrostatic repulsion force is produced, enabling to obtain the best friction effect due to the formation of a further thick water layer at a friction interface compared with a neutral gel.

[0043] The hydrogels of the invention preferably contain a plenty of water in order to obtain an enough low friction effect. The water content is preferably not less than 50 wt. %, in particular preferably not less than 100 wt. %.

[0044] The frictional coefficient of the obtained hydrogels is also preferably not more than 0.01, in particular preferably not more than 0.005.

[0045] As the form of the hydrogels of the invention, it may be either form if it is a form to contain the linear chain polymer chains in the polymer gels or on the polymer gel surfaces. However, in order to effectively realize the low frictional force, one having the linear chain polymer on the gel surface is preferable, further, one to which the polymer chains are graft-polymerized on the surface is preferable.

EXAMPLE

[0046] In the following, the low friction hydrogels of the invention are explained in more detail by the examples, the comparative examples and the test examples. However, the invention is not limited in any way by these.

Example 1

[0047] Methylenebisacrylamide 8% as a crosslinking agent, alpha-ketoglutaric acid 0.1% as a photosensitizer, and 4 g of poly(AMPS) of molecular weight 250,000 synthesized in advance were added to an aqueous solution (AMPS) 100 ml containing 2-acrylamido-2-methylpropanesulfonic acid 20 g, followed by carrying out 400 W UV irradiation to prepare AMPS gel containing the linear chain polymer on a glass plate.

Example 2

[0048] Except letting 8 g of poly(AMPS) of molecular weight 250,000 be contained in stead of 4 g in the example 1, AMPS gel containing the linear chain polymer was prepared in the same way as that in the example 1.

Comparative example 1

[0049] Except letting no poly(AMPS) of molecular weight 250,000 be contained, PAMPS gel containing no linear chain polymer was prepared in the same way as that in the example 1.

[0050] The gels of the examples 1, 2 and the comparative example 1 which were each cut out to the regular cube of one side 2 cm were placed on a glass plate, measuring the friction force in water by measurement of the shear stress in case of rotating the glass plate at various velocities. The results are shown in Table 1. Table 2 shows the results of the friction force in which the gels of the examples 2 and the comparative example 1 were each mounted not on the glass buton the same gels, measuring the friction force. TABLE 1 Between Gel-Glass Friction force Rate of rotation (radian) (Newton) 10 1 0.1 0.01 Example 1 0.04 0.005 0.0004 0.00002 Example 2 0.03 0.007 0.0008 0.0002 Comparative 0.1 0.04 0.02 0.01 Example 1

[0051] TABLE 2 Between Gel-Gel Friction force Rate of rotation (radian) (Newton) 10 1 0.1 0.01 Example 2 0.06 0.03 0.008 0.003 Comparative 0.3 0.04 0.03 0.02 Example 1

Example 3

[0052] Using dimethyl acrylamide (DMAA) instead of AMPS used in the example 1 and 2 g of linear chain poly(DMAA) of molecular weight 120,000 instead of poly(AMPS) of molecular weight 250,000, DMAA gel containing a linear chain polymer was synthesized in the same way as that in the example 1.

Comparative example 2

[0053] Except letting nolinear chain poly(DMAA) of molecular weight 120,000 in the example 3 be contained, DMAA gel containing no linear chain polymer was prepared in the same way as that in the example 3.

[0054] Using the gels of the example 3 and the comparative example 2, the results of the friction force between the gel and glass are shown in Table 3 in the same way as the example 1. TABLE 3 Between Gel-Glass Friction force Rate of rotation (radian) (Newton) 10 1 0.1 0.01 Example 3 0.03 0.01 0.007 0.005 Comparative 0.9 0.2 0.07 0.06 Example 2

Example 4

[0055] In the case of the usual AMPS gel synthesis in the comparative example 1, polymerization was made not on a glass plate but on a methacryllic resin plate to prepare a graft gel having free end poly(AMPS) chains on the gel surface. The obtained gel was swollen in the water to measure the friction force between the gel and glass in the same way as the example 1. The results compared with the comparative example 1 is shown in Table 4. TABLE 4 Between Gel-Glass Friction force Rate of rotation (radian) (Newton) 10 1 0.1 0.01 Example 4 0.02 0.006 0.0007 0.0001 Comparative 0.1 0.04 0.02 0.01 Example 1

Example 5

[0056] 10 g of PAMPS gel synthesized in the comparative example 1 was let stand in an aqueous solution 200 ml containing 4 g of poly(DMAA) of molecular weight 100,000 synthesized in advance at room temperature for 1 week to obtain PAMPS gel containing 0.6 g of poly(DMAA).

[0057] After the gel thus obtained reaches to the equilibrium swelling in pure water, the friction force was measured using a viscoelasticity tester (ARES, Rheometric Scientific, Inc.), showing that the friction force was reduced to {fraction (1/12)} compared with that containing no poly(DMAA) 0.6 g.

Example 6

[0058] An aqueous solution 100 ml containing 3 g of poly(acrylamide) of molecular weight 120,000, 7 g of polyvinyl alcohol of molecular weight 80,000 and 2 g of ethyleneglycol glycidyl ether as the crosslinking agent was heated at 70° C. for 24 hours to obtain polyvinyl alcohol gel containing poly(acrylamide). This gel was cut into the regular cube of 1 cm×1 cm×1 cm to measure the friction force by the method described in the example 5, showing that the value was 15% compared with that containing no poly(acrylamide).

Test example 1

[0059] Methylenebisacrylamide 0.5 g as a crosslinking agent, and alpha-ketoglutaric acid 0.1 g as a photosensitizer were added to an aqueous solution 100 ml containing AMPS 10 g, followed by carrying out 400 W UV irradiation to prepare AMPS gel on a glass plate. The obtained gel was immersed in an aqueous solution 100 ml containing AMPS monomer 4 g for 1 week to disperse the monomer into an inner part, followed by 400 W UV irradiation to polymerize the polymer in the inner part, thereby preparing a gel containing linear chain polymer chains.

[0060] Separately, in the case of the AMPS gel polymerization with the same composition as the above described gel, the polymerization was made on polystyrene in place of a glass plate to prepare a gel having poly (AMPS) graft chains on the surface.

[0061] The results of the friction force of the gels against a glass plate measured by the method described in the example 5 are shown in FIG. 1.

[0062] As the figure shows, contrasting to the fact that in the usual AMPS gel prepared on the glass plate was in the order of 10⁻¹-10⁻² Nm⁻², the gel containing linear chain polymer showed 10⁰-10⁻² Nm⁻², and the gel containing the linear chain graft chains on the surface prepared making polystylene a substrate showed a further small friction force as 10⁻¹-10⁻³ Nm⁻². The calculation of the frictional coefficients according to these values gives 10⁻²-10⁻³, 10⁻¹³-10⁻⁴, and 10⁻⁴-10⁻⁵ respectively. In the area small in a relative velocity, the presence of the linear chain polymer reduced the friction force in not less than two places.

Test example 2

[0063]FIG. 2 shows the results in which a load dependency of the friction force measured fixing the velocity at 0.01 rad/s.

[0064] According to the figure, the gel containing the linear chain polymer and the gel having graft chains on the surface both become small in the friction force by not less than one place. In particular, the effect was remarkable in the low loading area, demonstrating reduction of the friction force in not less than two places.

[0065] The friction force of the gel having graft chains on the surface reduced largely, and in particular, the effect was remarkable in the area of the low velocity and low load.

Test example 3

[0066] Methylenebisacrylamide 1 wt. % as a crosslinking agent, and alpha-ketoglutaric acid 0.5 wt. % as a photosensitizer were added to an aqueous solution 50 ml containing DMAA 7 g, followed by carrying out 200 W UV irradiation to prepare DMAA gel on a glass plate. AMPS gel was prepared in a similar method. In the meantime, an aqueous solution 100 ml containing ethyleneglycol glycidyl ether 2 g and polyvinyl alcohol 10 g was reacted at 80° C. for 24 hours, carrying out crosslinking to prepare polyvinyl alcohol (PVA) gel. Each gel obtained was immersed in an aqueous DMAA monomer solution of 1.0 M or an aqueous AMPS monomer solution of 1.0 M for 1 week to disperse the monomer into an inner part, followed by 400 W UV irradiation to polymerize the polymer in the inner part, preparing a gel containing linear chain polymer chains. FIG. 3 shows the frictional coefficient for each gel measured.

[0067] The upper figure are the results of the gels in which poly(DMAA) or poly(AMPS) was allowed to be contained in DMAA gel, and the lower figure are the results of the gels in which poly(AMPS) was allowed to be contained in polyvinyl alcohol (PVA) gel or AMPS gel.

[0068] In DMAA gel the frictional coefficient was reduced to one-tenth utmost due to the inclusion of linear chain polymer chains, in one in which poly(AMPS) was allowed to be contained in AMPS gel it was reduced to one-hundredth utmost, and in one in which poly(AMPS) was allowed to be contained in PVA gel it was reduced to one-thousandth utmost.

Test example 4

[0069] In the case of synthesis for the gels of DMAA and AMPS, polymerization was carried out being sandwiched not between glass plates but Teflon plates to prepare gels having free end graft chains on the gel surface. FIG. 4 shows the frictional coefficient for each gel measured.

[0070] In the figure, DMAA shows the gel polymerized by sandwiching between glass plates, DMAA graft does the gel polymerized by sandwiching between Teflon plates and having graft chains on the surface, PAMPS does APMS gel polymerized by sandwiching between glass plates, and PAMPS graft does APMS gel polymerized by sandwiching between Teflon plates and having graft chains on the surface.

[0071] In DMAA gel the frictional coefficient changed to one tenth utmost due to the presence of free end graft chains on the surface, and in AMPS gel it did to one-thousandth utmost, showing a low value of the frictional coefficient hitherto not obtained, that is, 6×10⁻⁵.

INDUSTRIAL APPLICABILITY

[0072] According to the invention, a low friction material in a degree which has not been found as yet can be prepared. 

1. A low friction hydrogel, wherein a linear chain polymer is admixed with or graft-polymerized to a polymer gel.
 2. The low friction hydrogel according to claim 1, wherein the linear chain polymer is graft-polymerized on a surface of the polymer gel.
 3. The low friction hydrogel according to claims 1 or 2, wherein monomers constituting the polymer gel and monomers constituting the linear chain polymer are the same kind of monomers.
 4. The low friction hydrogel according to any one of claims 1 to 3, wherein the frictional coefficient is not more than 0.01.
 5. The low friction hydrogel according to any one of claims 1 to 4, wherein the content ratio of the linear chain polymer relative to the total weight of the low friction hydrogel is 2-300 wt. %.
 6. The low friction hydrogel according to any one of claims 1 to 5, wherein the polymer gel is an ionic gel.
 7. Use of the low friction hydrogel according to claim 6 on surfaces of a solid and a biological tissue.
 8. A method for preparing the low friction hydrogel, wherein the linear chain polymer or monomers forming the linear chain polymer is admixed with and/or graft-polymerized to a polymer gel or monomers forming the polymer gel.
 9. The method for preparing the low friction hydrogel according to claim 8, wherein the linear chain polymer is admixed with and then graft-polymerized to the polymer gel.
 10. The method according to claim 8, wherein one or more species of monomers to form the linear chain polymer are admixed with and polymerized to the polymer gel, so as to involve the linear chain polymer into the polymer gel.
 11. The method according to claim 8, wherein the linear chain polymer are admixed with and polymerized with one or more species of monomers forming the polymer gel, so as to involve the linear chain polymer into the polymer gel.
 12. The method according to claim 8, wherein one or more species of monomers forming the polymer gel and one or more species of monomers forming the linear chain polymer are admixed and polymerized, so as to graft the linear chain polymer to the polymer gel.
 13. The low friction hydrogel according to any one of claims 8 to 12, wherein monomers forming the polymer gel and monomers forming the linear chain polymer are the same type of monomers.
 14. The method for preparing the low friction hydrogel, wherein monomers forming the polymer gel are polymerized on a hydrophobic substrate. 